Conventional remote center compliance (RCC) devices employ one or two stages or portions of movable or deformable elements to provide the necessary combination of independent translational and rotational compliant action. Two stage RCC devices are disclosed in U.S. Pat. Nos. 4,098,001, and 4,155,169. Typically, these two stage devices include one stage having a set of deformable elements which lie along radii from a center, and a second stage having a set of deformable elements which are either parallel to each other or generally transverse to the first set. Together, these two stages provide the translational and rotational compliance of the RCC device. Each stage typically includes deformable elements, shear pads of alternating elastomer and metal layers, or open-coil springs. Single stage RCC devices, for example U.S. Pat. No. 4,414,750, include a single stage having a set of deformable elements axially disposed along generatrices of a cone. Typically, the deformable elements are either open-coil springs or shear pads of alternating metal and elastomer layers. This single stage provides both rotational and translational compliance.
Although single stage RCC devices are simpler and less expensive to manufacture than the two stage devices, both the shear pad and open-coil spring RCC devices exhibit problems which have kept them from being widely used as automatic insertion aids. The shear pad RCC devices perform satisfactorily, but only within fairly small ranges of motion. If the pads are repeatedly subjected to the high shear strains resulting from relatively large misalignment between the RCC device and the workpiece, the elastomer layers tend to fatigue and tear. This fatigue causes the shear pads to eventually lose their integrity, thereby causing the RCC device to become ineffective. In addition, since shear pads are relatively expensive, single stage shear pad RCC devices are not cheap enough to be widely used in industry.
Single stage RCC devices employing conventional coil springs as deformable elements are generally less expensive to manufacture than the shear pad RCC devices, but their center of compliance does not project a useful distance from the device. Although an RCC device with a non-projected center of compliance does have some efficacy, it does not have the efficacy or characteristic remote center of compliance of a true RCC device. Thus, conventional coil spring insertion aids should not be considered true RCC devices, and are not effective for use in operations requiring the rotational and translational compliant characteristics of an RCC device.